The present invention relates to analog-to-digital conversion, and, in particular, to a method and apparatus of system offset calibration using an overranging analog-to-digital converter.
An analog-to-digital converter (ADC) system is configured to convert an analog input signal into a digital output signal. The ADC system comprises a programmable gain amplifier (PGA) and an analog-to-digital converter (ADC). The PGA is configured to adjust the amplitude of the analog input signal to match the input signal level to the full signal range of the ADC. An offset may be present in the input signal, while another offset may be caused by the PGA. These offsets may cause significant performance degradation in the ADC conversion process.
FIG. 1 is a graphic illustration of a system transfer curve for an ADC that includes a negative offset. The digital output code corresponds to zero when a voltage associated with the analog input signal is less than the magnitude of the negative offset. As a result, codes are lost for a xe2x80x9cdeadzonexe2x80x9d portion of the transfer curve.
FIG. 2 is a graphic illustration of a system transfer curve for an ADC that includes a positive offset. The digital output signal is clamped at a digital code that corresponds to a maximum code when the sum of the voltage that is associated with analog input signal and the offset exceeds the maximum range of the ADC. The digital output signal is saturated at the maximum code before the full voltage range of the analog input signal can be utilized such that the dynamic range of the ADC circuit is reduced.